With growing concerns about global environmental issues and energy issues, a solar cell has been paid attention as an energy generating means. Such energy is clean with no concerns about drying up. When a solar cell is used in outdoor environment such as on the roof of a building, it is generally used in the form of a solar cell module.
The aforementioned solar cell module is generally produced according to the following procedures. First, a crystalline solar cell element (hereinafter also referred to as the power generating element or cell, meaning the same) formed from polycrystalline silicon, monocrystalline silicon or the like, or a thin film solar cell element obtained by forming an ultra-thin (several micrometers) film made of amorphous silicon or crystalline silicon onto a substrate of glass or the like, is manufactured. Next, in order to obtain a crystalline solar cell module, a protective sheet for a solar cell module (surface protective sheet), an encapsulating material sheet for solar cell, a crystalline solar cell element, an encapsulating material sheet for solar cell and a protective sheet for a solar cell module (back surface protective sheet) are laminated in this order. On the other hand, in order to obtain a thin film solar cell module, a thin film solar cell element, an encapsulating material sheet for solar cell and a protective sheet for a solar cell module (back surface protective sheet) are laminated in this order. Thereafter, a solar cell module is manufactured through a lamination method in which the laminated material is absorbed under vacuum and pressed with heating. Solar cell modules manufactured in this manner are weather-resistant and thus are suitable for use in outdoor environment such as on the roof of a building.
As an encapsulating film material for a solar cell, a film made of an ethylene/vinyl acetate (EVA) copolymer has been widely used because it is excellent in transparency, flexibility and adhesiveness. For example, Patent Document 1 discloses an encapsulating film excellent in both adhesiveness and film-forming properties consisting of a crosslinking agent and an EVA composition containing trimellitate. However, when the EVA composition is used as a constituent material of an encapsulating material for solar cell, there is the risk of possibly affecting the solar cell element by the component such as acetic acid gas and other unwanted gas generated by decomposition of EVA.
On the other hand, there has been proposed the use of a polyolefin based material, particularly an ethylene based material, as an encapsulating film material, because it is also excellent in insulation properties (for example, see Patent Document 2).
Meanwhile, there has also been proposed a resin composition for an encapsulating material for solar cell using an ethylene/α-olefin copolymer excellent in a balance between rigidity and crosslinking properties, and extrusion moldability (for example, see Patent Document 3).